1. Field of the Invention
The present invention relates to an organometallic complex which can generate light emission from a triplet excited state to emit phosphorescence, and a light-emitting element containing the organometallic complex.
2. Related Art
Organic compounds (organic molecules) undergo a transition to an upper electronic state (excited state) when absorbing light. By way of the excited state, various reactions (photochemical reactions) may be occurred, or light emission (luminescence) may be generated. There are various applications of the organic compounds.
As an example of photochemical reactions, production and reaction of singlet oxygen can be nominated. Since the ground state of oxygen molecules is a triplet state, singlet state oxygen (singlet oxygen) is not produced by direct photoexcitation. However, in the presence of another triplet excited molecules, singlet oxygen is produced. In this instance, compounds capable of forming the foregoing triplet exited molecules are referred to as photosensitizer, and utilized as, for example, photobleaching agent or antimicrobial activity compounds.
As stated above, in order to produce singlet oxygen, photosensitizer which can form triplet excited molecules by photoexcitation is required. However, since the ground state of general organic compounds is a singlet state, the photoexcitation to a triplet excited state is a forbidden transition, that is, the probability of generating triplet excited molecules is very small (generally, singlet excited molecules are generated). Therefore, as the photosensitizer, compounds that are susceptible to give rise to intersystem crossing from a singlet excited state to a triplet excited state (or compounds that allow the forbidden transition that photoexcitation to a triplet excitation state) are required. In other words, such compounds are beneficial in using as photosensitizer Further, such compounds may emit phosphorescence.
In recent years, materials emitting phosphorescence have attracted attention as light-emitting substances used for a light-emitting element such as an electroluminescent (EL) element.
Phosphorescence is the light emission occurs from a transition between electronic states of different multiplicities, that is, the light emission occurs from the triplet excited state back to the singlet ground state. Further, fluorescence is the light emission that occurs from the singlet excited state back to the singlet ground state.
The compounds that can emit phosphorescence, that is, the compounds that can generate light emission from the triplet excited state (hereinafter, phosphorescent compounds) have high internal quantum efficiency of from 75 to 100% in theory. The value is approximately three to four times as large as that of compounds that emit fluorescence. Therefore, a high efficient light-emitting element can be manufactured by phosphorescent compounds.
However, many of materials that have been conventionally used as light-emitting substances emit fluorescence. Phosphorescent compounds are very few in number. Therefore, many chemists have developed the phosphorescent compounds. (For example, refers to Tetsuo Tsutsui et al. “Japanese Journal of Applied Physics”, vol. 38, L1502-L1504 (1999), and M. A. Baldo et al. “Nature (London)”, vol. 403, 750-753 (2000).)
In the above both references, an organometallic compound containing iridium as a central metals is disclosed. The organometallic compound is a type of organometallic complexes referred to as an orthometallic compound.
The structures of ligands of iridium complexes disclosed in the above both references are comparatively simple. The iridium complexes can give green emission with good color purity. In order to control the emission color, it is necessary to change the structure of the ligands. For example, M. Thompson et al. disclosed that they achieved some emission colors by synthesizing various ligands and iridium complexes containing the foregoing various ligands in “10th International Workshop on Inorganic and Organic Electroluminescence (EL '00)”, 35-38.
However, almost the foregoing ligands of iridium complexes are confined to those that can form five-membered rings with central metals. The present situation is that appropriate emission color is explored from the foregoing confined ligands. That is, the fact is problematic that applicable kinds of ligands are very few in number.
In addition, many of the foregoing ligands have difficulty synthesizing, and so the number of processes for synthesizing is large. As a result, the cost of materials is increased. In view of the cost, the yield of organometallic complexes is one of the important factors.
Further, problems have been arisen in case of using organometallic complexes for an electronic device such as a light-emitting element since organometallic complexes are susceptible to be decomposed and have poor heat resistance properties.
Accordingly, the development of organometallic complexes is required which can synthesize ligands easily, which can be synthesized with good yields, and which have excellent heat resistance properties.